Arthropod-borne (arbo) viruses, most importantly alphaviruses and flaviviruses, cause widespread epidemics of fever, encephalitis and arthritis and pose increasing threats to human populations through expansion into new geographic areas. Encephalomyelitis due to arbovirus infection of neurons is a particularly important global cause of morbidity and mortality because neuronal damage can lead to chronic disease and long-term disability, as well as acute fatal disease. There are no treatments for these infections and vaccines are not available for most. Alphaviruses that cause encephalitis (Venezuelan, western and eastern equine encephalitis viruses) infect neurons and are endemic in the Americas. Recovery from infection requires virus clearance from neurons and this poses unique challenges for the immune system. A noncytolytic process is needed to avoid irreversible neurologic damage and the process must be effective to avoid chronic or progressive neurologic disease. Our studies of the prototype alphavirus, Sindbis virus (SINV), in mice have shown that neurons are the primary target cells and that both virus and host factors determine outcome. Strains that cause acute nonfatal encephalomyelitis in weanling mice (e.g. AR339, TE) provide a system for studying the complicated process of recovery from neuronal virus infection. We have shown that infectious virus can be cleared by the combined effects of antibody (Ab) to the SINV E2 glycoprotein and interferon (IFN)-g, through processes that do not damage the infected neurons. However, preservation of these essential cells results in persistence of viral RNA in the central nervous system (CNS) and the need for long-term suppression of virus replication. Detailed study of virus clearance from the CNS of immunologically normal 4-6 week-old C57BL/6 mice over 6 months has revealed 3 phases of the clearance process: 1) clearance of infectious virus, but continued high levels of viral RNA; 2) gradual decrease in the amounts of viral RNA without production of infectious virus; and 3) maintenance of viral RNA at low levels with prevention of virus reactivation. We hypothesize that different components of the immune response are essential for each of these stages. We will define the mechanisms of staged recovery from SINV encephalomyelitis through the following specific aims: (1) Determine the components of the immune response that clear infectious virus from the CNS; (2) Determine the components of the immune response that decrease viral RNA in the CNS after infectious virus is cleared; and (3) Determine the role and maintenance of resident antibody-secreting cells in inhibition of virus reactivation.